test(sdk-coin-polyx): cover MPCv1 decrypt-failure error path

Add a test asserting the addRecoverySignature() MPCv1 branch
rethrows a descriptive 'Error decrypting user keychain' message
when bitgo.decrypt() rejects (e.g. wrong wallet passphrase).

The automated review dispatched after the prior test-coverage
commits flagged this catch/rethrow branch, moved out of recover()
and into addRecoverySignature() in the MPCv2 recovery commit, as
still unexercised by any test.

Ticket: WCI-1228
Session-Id: 3a00bebe-fe12-4aea-9f1a-5a2d0dd8e94b
Task-Id: dfadd58f-a04c-4dd9-819b-506439735cec

Author: vibhavgo

modules/sdk-coin-polyx/test/unit/polyx.ts

@@ -272,6 +272,16 @@ describe('Polyx:', function () {
         sandBox.assert.calledOnce(getTSSSignatureStub);
       });
 
+      it('should throw a descriptive error when user keychain decryption fails on the MPCv1 path', async function () {
+        sandBox
+          .stub(baseCoin as unknown as { isMpcv2SigningMaterial(): void }, 'isMpcv2SigningMaterial')
+          .resolves(false);
+
+        sandBox.stub(bitgo, 'decrypt').rejects(new Error('password error'));
+
+        await baseCoin.recover(mpcV2RecoverParams).should.be.rejectedWith(/Error decrypting user keychain/);
+      });
+
       it('should throw when commonKeyChain from MPCv2 keycard does not match bitgoKey', async function () {
         const mismatchedBitgoKey = mpcV2CommonKeyChain.slice(0, -8) + '00000000';
         const mismatchedParams = {